The invention relates to a gravimetric moisture-determination instrument that is cooled by an air stream. In essence, the instrument consists of a housing containing a balance and a radiant heater. The moisture content of a sample placed on the balance pan is determined by measuring the weight loss of the sample while it is being dried under the heater.
Instruments for the gravimetric determination of moisture content are known and commercially available. They share as a common operating principle that a sample whose moisture content is to be determined is placed on a balance that continuously monitors the weight of the sample while the latter is being heated, normally by a conventional infrared radiator or halogen radiator. As a result of the heating, moisture is evaporated from the sample, which causes a weight loss that is measured by the balance. A variety of arrangements are used for placing the sample into the moisture-determination instrument. In one arrangement, the entire balance including the weighing pan slides in and out of the housing. In another arrangement, the balance is installed in a fixed position in the housing while the heater can slide or swivel out of the way. In a third category of instruments, the heater is installed in a hinged top cover of the instrument, and the sample pan is made accessible by tilting up the cover of the instrument housing.
The balance, and especially the weighing cell of the balance, can be protected from heating up, if the instrument is equipped with a means for conducting an air stream through a space between the balance pan and the pan-supporting part of the weighing cell, as described in the Swiss patent application CH-A-689650. To achieve a thermal separation between the hot portions of the moisture-determination instrument and the weighing cell, ambient air is blown through a wide air duct below the weighing pan, for example by a fan installed in the housing of the instrument. To clean the air before running it through the cooling duct, the arrangement may also include a filter pad at the intake opening. The moisture-determination instrument as described in the aforementioned reference has a housing with a drawer holding a precision balance. To place the sample on the balance, the drawer is pulled out, and to perform the measurement, the drawer is pushed in. The means for conducting an air stream, e.g., an air duct, is preferably configured so that it slides in and out of the housing together with the balance. The air duct is divided into two sections, one of which is installed in the part of the moisture-determination instrument that contains the heater, while the other section is connected to the balance.
If the moisture-determination instrument of the foregoing description is used in a dusty or exceptionally dirty environment, there is a risk that the dust particles from the ambient air may clog up the filter pad in a very short time, so that filter pads have to be replaced on a frequent or even daily basis. The problem with replacing the filter pads is that users may neglect to do it, particularly if the filter pad is set in a holder that is complicated to unscrew, as is the case with commercially available moisture-determination instruments of the kind where the balance is fixed in the housing and the heater is movably mounted above the balance. If the filter pad is not replaced when needed, the clogged-up filter will obstruct the passage of air, so that the instrument is no longer cooled properly.
Although it is conceivable to use the instrument without the filter pad, this will over time lead to an accumulation of dust in the air duct which can make the cooling action less effective. This can make it necessary to clean the air duct from time to time in a complicated procedure requiring a partial disassembly of the instrument.